Measurement of a fluid pressure in a volume has been provided to have a particular added-value when such measurement is carried out non-intrusively. For example, for pipes conducting oil and/or gas it may be substantially advantageous to avoid the so-called contact measurements as a contact between an electrified sensor and an aggressive, such as combustible, medium may be dangerous.
An embodiment of a contact-less device for measurement of a gas pressure is known from U.S. Pat. No. 3,021,711. The known device comprises a thin mantle wall of magnetic material which closed at one end. A cylindrical enclosure wall is caused to vibrate using magnetic coils. Around the cylinder the outer housing forms a closed space whereto a pressure medium is admitted having a known pressure whereas inside the cylindrical enclosure a fluid is provided whose pressure needs to be determined.
During the vibration of the cylinder wall the cross-section area of the cylinder varies and if there is a difference of pressure between the internal and external surface of the cylinder wall the wall thus yield a work corresponding to the difference of pressure. As a result the difference of pressures influences the natural frequency of the cylinder.
It is a disadvantage of the known device that it has complicated structure and is not suitable for determining pressure of a flowing fluid having substantial volume and speed, such as those within the oil and gas pipes.
A further embodiment of a non-intrusive device for measuring a fluid pressure is known from EP 0 088 362. In the known device pressure within a cylindrical shell is measured by generating acoustic waves in the shell in a manner which minimizes the propagation of any appreciable vibrational energy to its mechanical support. In particular, the mode of the generated vibration is chosen from among the banded resonance vibrations which can be generated in a cylindrical shell, the chosen banded resonance having an even number of longitudinally extending nodes parallel to the portion of the shell within which the vibration exists. Stress variation in the cylindrical shell due to the contained pressure causes a change in the banded resonant frequency which is monitored as a measure of the pressure. For example, an electromagnetic movement detector may be mounted adjacent the shell for detecting movements from forced vibrations caused.
It is a disadvantage of the known device that the known device has limited sensitivity and is not suitable for measuring pressure in fluids having a plurality of phases. Also, pressure of turbulent flows may be less accurate with the known device.